U.S. Pat. No. 3,966,267 and 3,929,382, assigned to the assignee of the present invention, disclose vehicle anti-lock systems of the type providing a signal for releasing the braking forces on the wheels of a vehicle, under particular wheel operating conditions, to minimize wheel locking due to braking, as coved result in vehicle skidding.
Systems of this kind have provided a lock signal when deceleration detection circuitry, for example, senses that the speed of a braked wheel is decreasing at a rate substantially faster than is vehicle speed, indicating an impending wheel lock condition, for releasing the braking force and thereafter permitting the subject wheel to spin up toward vehicle velocity, whereafter braking force is again restored to continue slowing the vehicle.
Said systems also have included fixed bleed circuitry providing a lock signal when the average, for example, wheel speed decreases below a declining reference signal representing a desired rate of decline in the speed of the faster wheel, and hence the vehicle. This circuit is useful, under a light load or low coefficient of friction conditions where the deceleration logic lock signal may have inadequate time to release the brakes prior to abatement of the deceleration logic lock signal due to lockup (cessation of deceleration) of such wheel. Particularly, the fixed bleed logic overcomes this problem by furnishing its own lock signal before wheel lockup. The fixed bleed circuit thereafter terminates its own lock signal when the subject wheel has spun up to a desired fraction of the vehicle speed.
Such prior skid control systems have also provided differential wheel speed logic to produce a lock signal where the speeds of two wheels being monitored differ substantially, as during braking where one wheel is on ice and the opposed wheel is on dry pavement, and lockup of the former is to be avoided.
Reference is made hereto co-pending application Ser. No. 831,908, filed Sept. 9, 1977 entitled Anti-Wheel Lock System, and assigned to the assignee of the present invention. Such application is directed to improvements over prior anti-lock systems of the type exemplified by the above-mentioned two patents.
The present invention is directed to an anti-lock system, generally of the type including those of the patents and application above-referenced, but particularly directed to anti-lock control of independently rotatable wheels on more than one axle. A typical example of the present invention involves anti-lock control of four braked, individually rotatable wheels carried at the ends of a pair of vehicle axles (e.g. tandem axles). It will be understood that the term wheel is here used broadly to include the wheel assembly at an end of an axle, whether such wheel assembly includes one, or more, individual wheel (tire and rim) units.
Accordingly, the objects of this invention include provision of:
A lock control system capable of anti-wheel lock control of braked, independently rotatable wheels on more than one axle, and particularly, braked independently rotatable wheels at the ends of tandem axles.
A system, as aforesaid, in which four distinct wheel speed signals are obtained, each representing the instantaneous speed of a corresponding one of said wheels, and wherein high and low wheel speed signals each are selected from all four of such wheel speed signals, wherein the thus detected high speed and low speed wheels may be on the same axle, different axles, and in the latter instance, on the same or opposite sides of the vehicle, dependent on vehicle operating conditions at that time.
A system, as aforesaid, which uses the aforementioned high wheel and low wheel signals, for example, from four wheels, to produce an intermediate signal which may be used to at least generally represent the average of the speeds of the four wheels.
A system, as aforesaid, in which the resultant average wheel speed signal is a weighted average, the weighting being in favor of the high wheel speed signal.
A system, as aforesaid, in which deceleration detector response speed is enhanced by causing same to monitor the low wheel speed signal so as to timely detect impending possible lockup in one of the wheels of the tandem axle set and where the remaining wheels, and hence the average of all four wheel speed signals, reflects a satisfactory slip condition during braking.
A system, as aforesaid, which permits adaptation, to tandem axles, of a system similar to that of our aforementioned copending application, with minimal revision, addition of parts and extra cost.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.